In its strict sense, this species is generally tetraploid (2n = 116), but a diploid variant is known from Mo. Distinction from the much more common atropurpurea, a triploid (2n = 87), can be difficult, especially with small immature plants (Cranfill 1980; FNA 2; Y, W). P. glabella is widely scattered in Ky. but rare except perhaps along drier cliffs of the Palisades of Ky. Rv. and on dolomite cliffs around the Bluegrass, especially in BULL and NELS. Compared to atropurpurea, glabella has petioles and rachises glabrous or thinly hairy (versus densely hairy on upper surface). Its leaves are monomorphic (versus somewhat dimorphic with narrower fertile pinnae), 1-2 times compound (versus 1-3), the terminal pinnae sessile (versus often stalked), the basal pinnae of larger fronds on stalks 0-4 mm (versus 5-15 mm); pinnae usually gray-green (versus bright green) and less hairy. Both species are generally apomictic.